1. Field of the Invention
The present invention relates to a method for measuring phase current for an inverter control apparatus and an inverter control apparatus to which the measuring method is applied, and more particularly, to a method for measuring phase current for an inverter control apparatus using a single current sensor and an apparatus therefor, which is capable of changing a reference voltage for modulating a pulse width in a cycle of a triangle wave and compensating for the reference voltage changed in the same cycle, to thus measure current every cycle of the triangle wave without measuring current with error.
2. Description of the Background Art
In general, when a three-phase alternating current motor (a three-phase induction motor) is controlled using an inverter, a current sensor is used for current control or to protect the motor and an electrical load from over-current. The three current sensors are installed to correspond to the respective phases between the inverter and the three-phase alternating current motor and measure current that flows through the three-phase alternating current motor. Also, in the case that three-phase phase current is in balance, current of two-phases is measured using two sensors and the remaining current is calculated. Accordingly, three-phase output current is obtained.
Recently, a method of measuring phase current of each switching state of the inverter by installing a single current sensor between a DC(Direct Current) output circuit and the inverter and estimating the three-phase output current of the inverter according to the phase current of each measured switching state is provided. Accordingly, it is possible to reduce expenses in the manufacturing of an apparatus for measuring three-phases alternating current.
FIG. 1 is a block diagram showing a constitution of an inverter control apparatus including an apparatus for measuring current of each phase of three-phase alternating current using a single current sensor according to a prior art.
As shown in FIG. 1, the inverter control apparatus includes a converter 11 for converting alternating current from a three-phase AC(Alternating Current) power source 10 into direct current, an inverter 14 including a pair of switching devices Q1 and Q4, a pair of switching devices Q3 and Q6, and a pair of switching devices Q5 and Q2 in the respective phases, the inverter 14 for converting direct current from the converter into alternating current and providing the alternating current to a three-phase induction motor 16, a current sensor 12 for measuring current that flows through a line between the converter 11 and the inverter 14, an analog-to digital (A/D) converter 13 for converting an analog direct current value measured by the current sensor 12 into a digital current value, an inverter controller 15 for generating a pulse width modulation (PWM) signal using a reference voltage of each phase and a triangle wave on the basis of the digital measured value from the A/D converter 13, to thus control switching of the switching devices Q1 and Q4, the switching devices Q3 and Q6, and the switching devices Q5 and Q2 in the respective phases.
The operation of the inverter control apparatus using the current sensor according to the prior art of the above constitution will now be described.
The converter 11 receives the three-phase alternating current from the three-phase current power source 10 and outputs the direct current after rectifying and smoothing to the inverter 14. The inverter 14 converts the direct current into alternating current and outputs the alternating current to the three-phase induction motor 16. The value of the direct current that flows between the converter 11 and the inverter 14 (flows through a so-called a direct current link) is measured using the current sensor 12. Thus the measured analog direct current value is converted to digital data using the A/D converter 13 and is output to the inverter controller 15. The inverter controller 15 compares the digital current value from the A/D converter 13 with a command current value, calculates a new current command value obtained by compensating for a difference value between the two values, generates a PWM signal corresponding to the new current command value, and outputs the PWM signal to the inverter 14.
The PWM signal is a rectangular wave for comparing the direct current reference voltages of the respective phases U, V, and W with the voltage signal of a triangle wave and for turning on or off the switch device of a corresponding phase. That is, the inverter controller 15 compares the triangle wave voltage signal with the reference voltage signals Vu, Vv, and Vw of the respective phases as shown in FIG. 2. When the reference voltage signals Vu, Vv, and Vw of the respective phases are larger than the triangle voltage signal, the inverter controller 15 outputs a high level of square wave signal (refer to the waveforms of Q1, Q3, and Q5 of FIG. 2) for turning on the positive switching devices Q1, Q3, and Q5 of a corresponding phase and a low level of square signal (waveforms that are obtained by inverting the waveforms of Q1, Q3, and Q5 of FIG. 2 and are not shown) for turning off the negative switching devices Q4, Q6, and Q2 of a corresponding phase. When the reference voltage signals Vu, Vv, and Vw of the respective phases are smaller than the triangle wave voltage signal, the inverter controller 15 outputs a low level of square wave signal (refer to the waveforms of Q1, Q3, and Q5 of FIG. 2) for turning off the positive switching devices Q1, Q3, and Q5 of the corresponding phase and a high level of square wave signal (waveforms that are obtained by inverting the waveforms of Q1, Q3, and Q5 of FIG. 2 and are not shown) for turning on the negative switching devices Q4, Q6, and Q2 of the corresponding phase. In the positive and negative switching devices, when the direction, in which current flows out through the inverter 14 to the three-phases induction motor 16, is considered a positive direction and the direction, in which current flows in from the three-phase induction motor 16 through the inverter 14, is considered a negative direction, in the case where the positive switching devices Q1, Q3, and Q5 are turned on, current flows out to the three-phase induction motor 16. In the case where the negative switching devices Q4, Q6, and Q2 are turned on, current flows in from the three-phase induction motor 16 through the inverter 14.
Therefore, the switching devices Q1 through Q6 of the inverter 14 are turned on when the pulse width modulated square wave signal is at a high level and are turned off when the pulse width modulated square wave signal is at a low level. Accordingly, the switching devices are at a certain state among the 24 states of FIG. 3. When the positive and negative switching devices of an arbitrary phase are simultaneously turned on, the circuit of the phase is electrically shorted. Accordingly, the inverter 14 and the motor 16 are burnt out. Therefore, it is essential for the inverter controller 15 to control the positive and negative switching devices of the respective phases to not to be simultaneously turned on. Therefore, it is assumed that the positive and negative switching devices are not simultaneously turned on. The current Idc measured by the current sensor 12 flows through the three-phase induction motor 16 and is one among the 8 values in the right most column of FIG. 3.
The above will now be described in more detail with reference to FIGS. 1 and 3.
Referring to FIG. 3, in the states of the switching devices Q1 through Q6 in the first and the last rows, the positive switching devices Q1, Q3, and Q5 are turned on and the negative switching devices Q4, Q6, and Q2 are turned off or the positive switching devices Q1, Q3, and Q5 are turned off and the negative switching devices Q4, Q6, and Q2 are turned on. In this case, referring to FIG. 1, there is only a path, through which the current Idc flows out through the inverter 14, and there is no path, through which the current Idc flows in from the induction motor 16, or there is no path, through which the current Idc flows out through the inverter 14, and there is only the path, through which the current Idc flows in from the induction motor 16. Accordingly, a closed circuit is not formed. Therefore, the current Idc does not flow. Therefore, the current value measured by the current sensor 12 becomes zero.
Referring to FIG. 3, in the state of the switching devices Q1 through Q6 in the second row, the positive switching devices Q1, Q3, and Q5 are respectively turned on, on, and off and the negative switching devices Q4, Q6, and Q2 are respectively turned off, off, and on. In this case, referring to FIG. 1, a closed loop is formed such that the current Iu of the phase U and the current Iv of the phase V flow out to the motor 16 through the positive switching devices Q1 and Q3 and that the current Iw flows in through the negative switching device Q2 from the motor 16. Therefore, when it is considered when the amount of current that flows out to the motor 16 is the same as the amount of current that flows in from the motor 16, the amount of the current Idc is the same as the amount of the current Iw and the direction of the current Idc is inverse to the direction of the current Iw. Therefore, the current Idc is xe2x88x92Iw.
Referring to FIG. 3, in the states of the switching devices Q1 through Q6 in the third row, the positive switching devices Q1, Q3, and Q5 are respectively turned on, off, on and the negative switching devices Q4, Q6, and Q2 are respectively turned off, on, off. In this case, referring to FIG. 1, a closed loop is formed such that the current Iu of the phase U and the current Iw of the phase W flow out to the motor 16 through the positive switching devices Q1 and Q5 and that the current Iv flows in from the motor 16 through the negative switching device Q6. Therefore, when it is assumed that the amount of the current that flows to the motor 16 is the same as the amount of the current the flows from the motor 16, the amount of the current Idc is the same as the amount of the current Iv and the direction of the current Idc is inverse to the direction of the current Iv. Therefore, the current Idc is xe2x88x92Iv.
Referring to FIG. 3, in the states of the switching devices Q1 through Q6 in the fourth row, the positive switching devices Q1, Q3, and Q5 are respectively turned on, off, and off and the negative switching devices Q4, Q6, and Q2 are respectively turned off, on, and on. In this case, referring to FIG. 1, a closed loop is formed such that the current Iu of the phase U flows out to the motor 16 through the positive switching device Q1 and that the currents Iv and Iw flow in from the motor 16 through the negative switching devices Q6 and Q2. Therefore, the current Idc that flows to the motor 16 is Iu.
Referring to FIG. 3, in the states of the switching devices Q1 through Q6 in the fifth row, the positive switching devices Q1, Q3, and Q5 are respectively turned off, on, and on and the negative switching devices Q4, Q6, and Q2 are respectively turned on, off, and off. In this case, referring to FIG. 1, a closed loop is formed such that the current Iv of the phase V and the current Iw of the phase W flow out to the motor 16 through the positive switching devices Q3 and Q5 and that the current Iu flows in from the motor 16 through the negative switching device Q4. Therefore, when it is assumed that the amount of the current that flows to the motor 16 is the same as the amount of the current that flows from the motor 16, the amount of the current Idc is the same as the amount of the current Iu and the direction of the current Idc is inverse to the direction of the current Iu. Therefore, the current Idc is xe2x88x92Iu.
The states of the switching devices in the sixth and seventh rows and the fact that the value of the current Idc is Iv and Iw can be understood by the above description. Therefore, description thereof will be omitted.
As mentioned above, in the states of the switching devices in the first and last rows, where the positive switching devices Q1, Q3, and Q5 are simultaneously turned on or the switching devices Q4, Q6, and Q2 are simultaneously turned on, the current Idc does not flow. However, in the states of the switching devices in the remaining 6 rows, the current Idc coincides with the current of a phase among the three-phase output current of the inverter 14.
Therefore, the current sensor 12 measures the current Idc and outputs the measured value to the A/D converter 13 as an analog value. The inverter controller 15 determines the values of the currents Iu, Iv, and Iw of the respective phases on the basis of the digital current value from the A/D converter 13 and the values such as xe2x88x92Iw and xe2x88x92Iv of the current Idc in the previously stored states of the switching devices Q1 through Q6 as shown in the table of FIG. 3. Therefore, the measurement of the currents of the respective phases, which flow through the motor 16, is completed.
It is possible to correctly measure the current Idc only when the switching control signal of the switching devices A1 through Q6, that is, the PWM signal from the inverter controller 15 is maintained not to be varying for a conversion time, where the A/D converter 13 converts the analog value of the current Idc measured by the current sensor 12 into a digital value. That is, it is possible to correctly measure the current Idc only when the switching states of the switching devices Q1 through Q6 are maintained as one among the 8 states of FIG. 3 for a predetermined minimum time or more. The conversion time commonly requires a switching maintaining time of 2 through 9 microseconds or more.
However, when the minimum switching maintaining time is shorter than the time spent on converting the analog current value sensed by the current sensor 12 into a digital value by the A/D converter 13, it is not possible to correctly measure the current. For example, in FIG. 2, the positive switching devices Q1, Q3, and Q5 are respectively turned on, off, and on (the state in the third row of FIG. 3) for the time Tp. It is possible to correctly measure the current Iv of the phase V only when the time Tp is larger than the minimum conversion time Tp_min of the A/D converter.
Therefore, in order to secure the minimum switching maintaining time of the inverter 14, a time, for which the current of a phase flows, is increased by changing (distorting) an original reference voltage Vu by dV. Therefore, the current value of each phase, which flows through the motor 16, is measured by the current sensor 12 for a cycle of the triangle wave by the changed reference voltage Vu_c. However, because the reference voltage is changed (distorted) by dV in order to secure the switching time, the measured current value is erroneous. Thus distorted current waveform of a phase is an alternating current waveform distorted as shown in FIG. 4 when the current waveform is measured by a measuring instrument such as an oscilloscope.
Therefore, when an original reference voltage is changed in order to secure the minimum switching time in one cycle of the triangle wave as mentioned above, the inverter controller 15 must output the PWM signal for compensating for the current to the inverter 14 by gradually compensating for the reference voltage during various cycles of the triangle wave generated hereafter in order to correct the distorted current. However because it is not possible to secure the minimum switching time during such compensating cycles, it is not possible to measure the current value.
Therefore, according to the conventional inverter control apparatus, it is not possible to obtain a fast current control characteristic when it is necessary to instantaneous current control.
Therefore, an object of the present invention is to provide an inverter control apparatus, in which measured current is not distorted and which is capable of instantaneous controlling current in measuring current in each phase using a single current sensor and a method for measuring phase current of an inverter control apparatus.
To achieve these and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described herein, there is provided an inverter control apparatus, comprising a converter for converting alternating current from a three-phase alternating current power source into direct current, an inverter provided with a pair of switching devices in each of the three phases, the inverter for converting direct current from the converter into alternating current and providing the alternating current to a three-phase induction motor, a current sensor for measuring current of each phase, which flows through the motor, an A/D converter for converting an analog measured value of current of each phase, which is measured by the current sensor, into a digital measured value, and an inverter controller for generating a reference voltage of each phase and a PWM signal using a triangle wave on the basis of the digital measured value from the analog digital converter, controlling switching of a switching device of each phase, changing the reference voltages of arbitrary two phases in one cycle of the triangle wave, and compensating for the amounts of changes in the changed reference voltages of the two phases in the one cycle of the triangle wave.
There is provided a method for measuring current of each phase for an inverter control apparatus comprising a converter for converting alternating current from a three-phase alternating current power source into direct current, an inverter comprising a pair of switching devices in each phase, the inverter for converting direct current from the converter into alternating current and providing the alternating current to a three-phase induction motor, a current sensor for measuring current of each phase, which flows through the motor, an analog-to digital (A/D) converter for converting an analog measured value of current of each phase, which is measured by the current sensor, into a digital measured value, and an inverter controller for generating a reference voltage of each phase and a pulse width modulation (PWM) signal using a triangle wave on the basis of the digital measured value from the analog digital converter, to thus control switching of a switching device of each phase, the method comprising the steps of (a) changing the reference voltages of arbitrary two phases in one cycle of the triangle wave, (b) measuring current of each phase for one cycle of the triangle wave using the current sensor, and (c) compensating for the amount of change in the reference voltages of the two phases in the one cycle of the triangle wave.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.